Cantilevered Davits - Engineering Problem

Discussion in 'Boat Design' started by oceannavigator2, Mar 22, 2014.

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  1. rxcomposite
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    rxcomposite Senior Member

    AH- Before the fire, there is sunlight which softens the composite and reduce its mechanical properties. There is also creep, caused by prolonged suspension of the load plus it is not always vertical load. It can swing.

    Aaah. Talk about sleepless nights and FoS.
     
  2. daiquiri
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    daiquiri Engineering and Design

    Oh, sorry then. The gray (grey?) areas looked like a foam core. :)
     
  3. rxcomposite
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    rxcomposite Senior Member

    Just lack of sufficient pattern to shade in Excel. :D

    But I identified it in words.
     
  4. oceannavigator2

    oceannavigator2 Previous Member

    Steel it is!

    So best way to make the steel into a smoooth, flat/matte black color and give myself many years until the rust?

    I don't have to sand blast it do i?
     
  5. Rastapop
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    Rastapop Naval Architect

    Different composites have different properties of course, but in a word: no.
    Steel is the heaviest solution for a given strength.
    Aluminium or composites will be bulkier but lighter for the same strength. (Although as the steel version will include a lot of empty space most of the extra bulk could be hidden anyway)
     
  6. groper
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    groper Senior Member

    Rx your table shows eglass UD fiber at 40Gpa modulus at 60% vf. This is normal. The manufactured rhs section daiquiri linked to uses half this value. My only conclusion is that it's not made from UD fibers but rather something else such as chopped fiber in random orientation or such like. This is why it's performance is poor for a beam application ....

    ON2 if you want to hear my solution, read my previous post. I gave you the details of what I'd use and the expected deflection. Strength is not as issue with this design, only stiffness using eglass. ..
     
  7. Rastapop
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    Rastapop Naval Architect

    Only for the same dimensions. When you increase the dimensions to match the strength, stiffness at the given load improves as well.
    Matching strength with carbon fibre (for example - not suggesting it as a material here) would give you about 80% of the stiffness of steel.
    Keep going thicker and your strength and stiffness both surpass the steel version, while still being far, far lighter.

    The number one reason steel is still popular these days is because it's so cheap. There are other reasons too (it's easy, etc), but that's the primary one.

    You realise that the entire vessel is composite anyway, right?
    And, if it wasn't for the fact the OP has some spare composite materials sitting around going to waste, that steel is the cost cutting option?
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    You realise that the fuel tank is exposed to the sun, heat and that is has no protection from the environment, other than the tank itself, and no protection from puncturing, spillages etc?

    If so, what does the hull material have to do with it? Could be made from Switz cheese cheese, the risk of the aforementioned remains the same.

    Cutting cost is related to risk mitigation.
     
  9. oceannavigator2

    oceannavigator2 Previous Member

    Wow. You would think the math would reveal the truth, but when the physical properties going into the equation and the math itself can't even be agreed upon, how are we to move forward?

    There is surely a correct answer here.

    Surely a point where composites are equal to steel.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    That's the point. Equal in what sense??

    Design is so much more than just looking at a set of numbers. Only those with a frame of reference of numbers alone will say otherwise. As comparing one sales sheet of values against another missing the whole point of what design is...and ones perspective of what the SOR is and experienced based upon previous said items etc etc....

    Thus, what is equal, other than the final objective?....very little.
     
  11. Barry
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    Barry Senior Member

    Back in post 40, Ocean asked about material for the posts that will hold up the cantilevered beam.

    So another weight and cost issue comes up.

    If the current roof and POST system could hold up the cantilever and the weight of the RIB, then running the beam back to the bulkhead seems like a good idea.

    But if Ocean has to pay for posts in any case, these could be considered and the material that was going to run the length of the roof can be omitted.

    Instead the davit will look like an upside down L with say a 6 foot vertical post and a 40 inch leg so the Rib will clear the post.

    I would choose either aluminum or stainless. Corrosion with steel and the maintenance that will be required in the future adds to the cost. Galvanized is a little better only if it is coated inside and out and you do not weld it. The weld will need to be protected so you are back to painting, at an additional cost at the upset because galvanized metal does not look good against the backdrop of a cat, and the other issue of welding galvanized metal without proper breathing protection. "don't try this at home"

    A quick calculation for the size of square tubing needed to make the davit work and then some discussion on the attachments

    Oceans from the USA so I will use imperial units

    The worst case scenario with a boat full of water might get the total weight up into the 2000 pound range so I will run the numbers based on the 1000 pounds per single davit and Ocean can decide if that is adequate.

    An 6061 T6 4 inch by 4 inch with a .250 inch wall will carry this load and take the stress up to about 9000 psi, which is under the yield strength of 11,000 psi for welded
    6061 T6

    Note the Yield strength for 6061 T6 is much higher than 11,000 but when welded the yield strength drops to close to the 0 temper.

    4 inch aluminum weighs 4 1/2 pounds per foot so each davit with the post weighs about 45 pounds. The cost of 45 pounds of 6061 should run about $112 each

    This would technically give you a factor of safety of 11,000 divided by 9,000 or 1.22
    with a fully water laden boat.

    Say Ocean can handle a 5 inch cross section with a 1/4 inch wall. The max stress will be 5,500 psi which will give a factor of safety of 2 with 2000 pounds of boat and water hanging off the back.

    The weight of the 5 inch square will be 56 pounds each and cost about $140 per davit but this includes the cost and weight of the post which up to now was not included

    The two issues will be 555 pound pull on the roof per davit to resist the moment caused by the boat. The back of the davit post will be attached to the roof as it is supporting it so this attachment must be able to take this load.

    The second will be the ability of the floor to take the load, but this would be an issue with either design

    A complete set of calculation would have to be done because the numbers above do no include the stresses caused by the vertical shear in the beam but they should be negligible, the stability of the post, but again this should be negligible
     
  12. rxcomposite
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    rxcomposite Senior Member

    Groper- I dont remember D posting a different value. He posted this.

    And I posted the values from Performance Composites since it tallies with Filament Winding composites data and my data. The original won't upload so I made a Jpeg copy. Not much difference except that Fiberline UTS is much higher. Since this is their propriety technology, it is possible they (Fiberline) is using a high tensile strength resin or Eglass. E glass minimum is 2,000 MPa. some have up to 3,500 MPa.

    What I pointed out is to use the yield point to avoid the laminate from cracking prematurely.
     
  13. oceannavigator2

    oceannavigator2 Previous Member


    This is what I'm saying: The objective is to hold a tender up with cantilevered davits. Surely as you continue to increase a fiberglass beam's scantlings, possibly to the point of absurdity, it starts to behave exactly like steel...

    If i take a 1/2" steel rod and hang a gallon/liter container full of water off the end, it will stay put, not deflect, not break. If i take any one of my composite, cored offcuts here in the shop, of 1" or 2" width and hang that same gallon/liter of liquid from it, there will also be no deflection, no breaking, no issues.

    So...the same is true of davits.
     
  14. oceannavigator2

    oceannavigator2 Previous Member

    Wow!!!

    Where did Barry come from?

    Absolutely brilliant solution. Definitely on the list. The support of the "floor" is actually the second strongest spot on the boat. It is the aft box beam of a catamaran. Often, davits are stuck into this beam and protrude at 45° to the water. Nothing but this beam holds the entire weight of the dinghy in conventional applications.




     

  15. groper
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    groper Senior Member

    No what i meant was, the fiberline RHS section data sheet quotes the E @ 20Gpa. Obviously their RHS GRP structural sections are not made from UD fiber...

    Every design done in composite should be completely custom to make use of the beauty of composite construction. That is, control of the fiber quantity, control of the fiber and or fabric type, control of the matrix properties, control of fiber orientation, tailored to the specific application in question.
    If you dont bother to design specifically for it, then composites are not an efficient way of achieving an outcome compared to say, aluminium or steel etc. The flip side is also true tho, if you do bother to specifically design it for a given purpose, then it can be far more efficient than any other material... (not that you didnt already know this RX, more for anyone else reading)
     
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